In the present description the aluminum alloys will be designated by the known international items of "Aluminium Association" ("Registration record of international alloy designations and chemical composition limits for wrought aluminium alloys"), and the chemical compositions of said alloys will be expressed (using known chemical symbols) by indicating the amounts of alloying elements in percent (%) by weight, being understood that the balance to 100% is represented by aluminium. Alloys containing an aluminium amount higher than 80% are to be considered as aluminium alloys.
It is known that in the construction of civil and military armored vehicles a very important role is played by the aluminium alloys having compositions specifically suitable for the manufacture of armor plates endowed with a high resistance to penetration by projectiles and to spalling.
During the second world war, alloy 2024-T4 of the Al-Cu-Mg series (4.5% Cu, 1.5% Mg, 0.6% Mn) was widely employed in the form of low thickness sheets as material for armor for protecting the pilots of the fighters and bombers. At that time the armor of the troop transport cars and of the combat cars was made exclusively of steel. Plates of great thickness made of light alloy, which would have been necessary for these uses, were used exclusively in the aircraft industry.
Ballistic tests performed in the U.S.A. at the beginning of the second world war (Trook B., "Military Applications of Aluminium", Metals Mater., VI, pages 263-265, June 1972), had proved how the use of aluminium alloys made it potentially possible to reduce by 28% the weight in respect of the use of a common steel for armors RH (with not hardened surface), if the ballistic properties of 19 mm thick plates of 2024-T4--known at that time--could be transferred to plates of the same alloy with a higher thickness (the parameters relating to the "ballistic properties" will be better defined hereinafter).
Successively, the ballistic behavior of plates obtained with other aluminium alloys (besides with said alloy 2024) having thicknesses varying from 38 to 102 mm, was studied comparing it with the behavior of a steel for armor RH. Weights being equal, the aluminium alloys exhibited a better behavior, as compared with said steel, to the projectiles of little firearms and to the spalls of disruptive projectiles, but the plates of greater thickness showed a brittle behavior to the projectiles of larger caliber. At that time, however, weldable aluminium alloys having mechanical resistance characteristics equivalent to the ones of the alloys experimentally tested were not available on a commercial scale. The interest for the aluminium alloys as materials capable of replacing steel in the armored vehicles rapidly dropped as a consequence.
Problems of military strategy arose during the Korean war and involved the necessity of having at disposal light air-transportable and parachutable armored vehicles. Therefore a new interest was directed to the light alloys and particularly to the alloys having a high mechanical strength such as the types 7075 (5.6% Zn, 2.5% Mg, 1.6% Cu, 0.22% Cr), 2024 and 2014 (4.4% Cu, 0.8% Si, 0.4% Mg, 0.8% Mn).
The physical-mechanical and ballistic characteristics of these alloys (the production technology thereof had undergone meanwhile a constant progressive improvement as compared with their first appearance rendered them potentially suitable for armors; furthermore, the relevant plate thickness necessary to provide the required ballistic protection were such as to ensure a sufficient structural rigidity to the armored vehicle with out having to use ribs or any additional supporting structural elements.
In this manner the original main object was of course achieved, namely a reduction in the vehicle weight; a lowering in the production costs was attained too. However, the above-mentioned alloys exhibited considerable technological limitations consisting, on one side, in their low weldability and, on the other side, in the excessive degradation of their mechanical properties at the welded joints along with a marked tendency to spalling especially in the case of the higher plate thickness. On the other hand, the assembly by means of riveting could not be allowed as the rivets may become just secondary projectiles when they are hit by a projectile or by projectile spalls.
It was then necessary to have recourse to weldable and less brittle aluminium alloys: the choice fell on binary alloys Al-Mg which, besides excellent weldability characteristics, high toughness and corrosion resistance properties and medium strength characteristics, exhibited sufficient ballistic properties.
These alloys--belonging to type 5083 (4.5% Mg, 0.7% Mn, 0.15% Cr)--have constituted for a certain period of time (in the known "H115 temper") the only light material utilized in the armored vehicles for troops transportation; their most important application occurred in tracked vehicle M-113, designed and developed in U.S.A. by Food Machinery Corp. (San Jose, Calif.).
The use of aluminium alloys was taken into consideration also when the idea of constructing the air-transportable amphibian assault tank M 55I equipped with a steel turret and with a light alloy hull was carried into effect in the U.S.A.
In relation to these utilizations it appeared then necessary to obtain weldable aluminium alloys with ballistic properties superior to the ones of the Al-Mg alloys.
Thus, ternary alloys Al-Zn-Mg, studied and developed in Europe and U.S.A. simultaneously began to be used.
In respect to the Al-Mg alloys, the Al-An-Mg alloys--the application of which in the construction of hulls for armored vehicles has been extending more and more in the last years--result more easily hot workable, do not require (since they are heat-treatable alloys) any final cold deformation to increase the mechanical strength (what is necessary, conversely, for the Al-Mg alloys) and exhibit, for certain Zn and Mg contents, higher values of mechanical strength, chiefly in the welded manufactured articles in relation to the possibility of recovering, to a considerable extent, merely by natural aging, the mechanical characteristics worsened in consequence of the welding heating. Said alloys exhibit also better fatigue strength and, as already mentioned, higher ballistic properties than the Al-Mg alloys.
A wider application in the field in question have found Other suitable alloys include and 7005 (4.5% Zn, 1.4% Mg, 0.13% Cr, 0.45% Mn, 0.14% Zr) in U.S.A. alloy 7020 (4.5% Zn, 1.2% Mg, +Cr, +Mn, +Zr, similar to U.S.A. type 7005) in Europe and--limitedly in the United Kingdom--alloy 7017 (5.0% Zn, 2.5% Mg, 0.13% Cu, 0.15% Cr, 0.3% Mn, 0.15% Zr, similar to alloy 7039, in respect of which alloy 7017 exhibits a slightly higher mechanical strength).
Alloy 7039 provides, among the Al-Zn-Mg alloys and for certain plate thicknesses, one of the best combinations of resistance to penetration (such property being essentially bound to the tensile characteristics and to the hardness of the material) and of resistance to spalling (such property being bound to the material toughness).
For some applications, however, the use of alloy 7039 has not proved thoroughly satisfactory due to the not high ballistic resistance to the conventional and armor-piercing projectiles which hit the plate at angles approximately from 30.degree. to 55.degree. in respect of a line normal to the plate surface. For angles comprised in said range, the high-hardness steel for armors IT80 shows a higher ballistic resistance, on equivalent weight basis (weight per unit area) (REKER F. J., "Anwendung von Aluminium bei gepanzerten Militarfahrzeugen". Aluminium, LIII, pages 421-426, July 1977). Furthermore, for high thicknesses, the plates made of alloy 7039 exhibit tendency to spalling.
Aluminium alloys with mechanical strength and hardness characteristics higher than the ones of the previously cited Al-Zn-Mg alloy generally exhibit, as mentioned hereinbefore, an excessive tendency to spalling, independently of the plate thickness.
H. P. George and H. W. Euker ("Impact Resistant Aluminium Alloy Plate", U.S. Pat. No. 3,042,555, July 3, 1962), say that it is possible to overcome the problem of spalling by overageing the inner surface (opposite to the projectile impact surface) of plates made of Al-Zn-Mg-Cu alloys 7075 and 7178 (6.8% Zn, 2.7% Mg, 2% Cu, 0.22% Cr). Such a process, however, is very difficult to control, and although the plate obtained exhibits an improved resistance to spalling, such improved property is achieved to the detriment of the resistance to penetration.
M. C. Fetzer, J. D. Sprowl and W. R. Mohondro ("Aluminium Composite", U.S. Pat. No. 3,649,227, Mar. 14, 1972) state that the problem of increasing the resistance to penetration by armor-piercing projectiles and the resistance to disruptive projectile splits whilst simultaneously attaining a good resistance to spalling (comparable to or higher than the one of alloy 7039) can be solved by means of composite plates consisting of two layers "welded" to each other by hot rolling, the outer layer ("front") being made of an aluminium alloy endowed with higher mechanical resistance and hardness characteristics than the ones of alloy 7039, the inner layer being made of an alloy having a lower mechanical resistance than the alloy of the outer layer, but tougher; the composite plate is characterized also in that the thickness of the inner layer ranges from 4 to 20% (preferably from 5 to 12%) of the total thickness of said composite plate. The outer ("front") layer and the inner layer are separated respectively from alloys having a tensile strength of 446-618 N/mm.sup.2 and 309-480 N/mm.sup.2 (Newton/mm.sup.2) respectively.
Said patent discloses the utilization, for the front layer, of an alloy of series 7000 (containing also copper up to 3%, such as for example types 7075 and 7178), preferably of the alloy containing 6.6% Zn, 2.5% Mg, 0.15% Cu (+Mn, +Cr, +Zr, +Ti), and for the inner layer the utilization of an alloy also of type Al-Zn-Mg (with up to 3% of copper), preferably of the alloy containing 4.5% Zn, 0.75% Mg, 0.15% Cu (+Cr, +Mn, +Zr, +Ti); it is an essential condition that the inner layer should have a tensile strength lower than the one of the front layer by at least 15 N/mm.sup.2, preferably by 49 N/mm.sup.2.
The composite plate according to the aforesaid patent exhibits undoubtedly a ballistic behavior superior to the one of the one-layer plates consisting of any of the individual alloys mentioned for the said composite plate. However, said composite plate suffers from severe limitations of use when its front layer is prepared from Al-Zn-Mg-Cu alloys with a very high mechanical strength (types 7075 and 7178) in relation to the unsatisfactory behavior of said alloys to welding, due to a high embrittlement degree of the welded joint.
On the other hand, even if the welding of the plates in question should be limited to their inner layer (independently of the presence of not of Cu in the alloy forming said layer), the thickness considered for said layer would be insufficient to ensure welded joints having a satisfactory mechanical strength, and furthermore, should the alloy having the above-cited preferred composition be utilized for the front layer, then it would be possible, on one side, to effect a whole thickness welding of the composite two-layer plate, but, on the other side, serious problems of resistance to stress corrosion cracking would arise, in particular in the jointing area, due to the high content, in said preferred alloy, of the percentage of Mg and of the percentage of Zn+Mg. Such drawbacks imputable to stress corrosion could be reduced by subjecting said material to very severe ageing treatments, which, however, would result in an excessive worsening of the mechanical properties, and by consequence of the ballistic resistance of the plate.